A frame structure for a backrest, particularly a backrest of a vehicle seat, is expected to be lightweight to make the whole vehicle reduced in weight. Also, it is expected to have a high enough stiffness to properly protect passengers by enduring the moment generated by a great load of passengers or the like in the case of lateral or rear-end vehicle collision.
If such a frame structure is chiefly made of metal, the frame structure would have a comparatively complicated shape as disclosed in JP 2010-94436-A and, therefore, members of the frame would need to be bonded with many processes such as welding to meet the above-described demands. Besides, the structure chiefly made of metal cannot be reduced in weight so greatly.
On the other hand, FRP (Fiber-Reinforced Plastic) is a lightweight and high stiffness material known as an alternative to metal. Some prototypes of frame structure for a backrest have been made of FRPs as disclosed in JP 2005-194-A, JP 2010-500198-A and JP 2010-220748-A. However, it may be difficult that the structures disclosed in JP '194, JP '198 and JP '748 are formed with reinforcing fibers disposed at appropriate positions because the main structure parts of the frame structure are made of the FRP substantially as a whole or the reinforcing fibers exist inside the thickness of the main structure parts. Structures made of CFRP (Carbon Fiber-Reinforced Plastic) as disclosed in JP '194, JP '198 and JP '748 among FRPs would use many pieces of the expensive CFRP and, therefore, the production cost would increase. Further, it may be difficult for a whole of the frame structure disclosed in JP '194 to be formed as an integral structure because some portions of the frame structure have relatively complicated shape with unevenness and ribs to enhance the stiffness. As a result, there is a room for improvement in producing a lightweight and high stiffness frame structure at cheaper prices with ease.
Accordingly, it could be helpful to provide a frame structure and its manufacturing method at a cheaper price with ease, particularly by applying a new method to set a predetermined portion which should be reinforced with an FRP sheet.